1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus.
2. Description of the Related Art
In recording electrophotographic images, image forming apparatuses using laser light have been widely used. The image forming apparatuses have an optical scanning apparatus that scans the front surface of a photosensitive drum with laser light and forms a latent image on the front surface of the drum.
The optical scanning apparatus has a light source, an optical system at the front of a deflector, a rotational polygon mirror, a scanning optical system, and the like. The laser light emitted from the light source is incident on the rotational polygon mirror via the optical system at the front of the deflector and guided to the drum via the scanning optical system after being deflected by the reflecting surface of the rotational polygon mirror. Note that the reflecting surface of the rotational polygon mirror is also called a “deflecting and reflecting surface”.
A method for causing the laser light to be incident on the rotational polygon mirror includes an under field type and an over field type. In the following description, the under field type and the over field type will be called an “UF type” and an “OF type”, respectively, for the sake of convenience.
According to the UF type, the width of incident light is smaller in size than the length of a deflecting and reflecting surface in a direction corresponding to a main-scanning direction (see, for example, Japanese Laid-open Patent Publication No. 2005-92129). In this case, the incident light is entirely reflected by the deflecting and reflecting surface. In addition, a reduction in the area of a deflecting and reflecting surface has been attempted to increase the speed of optical scanning (see, for example, Japanese Laid-open Patent Publication No. 2010-122248).
According to the OF type, the width of incident light is greater in size than the length of a deflecting and reflecting surface (see, for example, Japanese Laid-open Patent Publication No. 10-206778). In this case, the peripheral portion of the incident light is not incident on the deflecting and reflecting surface.
In recent years, there has been a great demand for the higher speed of image formation and the higher quality of images in image forming apparatuses.
However, with the conventional image forming apparatuses, it is difficult to achieve desired high speed and high quality without increasing their sizes and costs.
Therefore, there is a need to perform optical scanning on a scanning surface at high speed and with high accuracy without increasing a size and a cost.